A. Field of Invention
This invention pertains to a method of making vascular prostheses from expanded PTFE (ePTFE) such as grafts and stent liners, said prostheses having an increased diameter, and a decreased wall thickness as compared to prior art grafts so that they can be used as a prosthesis in blood vessels having a relatively large diameter such as the aorta. More particularly, the invention pertains to a method of making a graft formed of at least one or more layers of ePTFE having an extremely thin wall yet high longitudinal tensile strength. A stent may be used in conjunction with the ePTFE layers, which allows the resulting prosthesis graft to be implanted without the use of a balloon or other graft expanding means.
B. Description of the Prior Art
Studies have shown tubes made of expandable polytetrafluorethylene (ePTFE) are ideally suited for various devices such as vascular prostheses. Vascular prostheses can be used to replace or repair blood vessels. Tubes made of ePTFE exhibit superior biocompatability, and can be made with a variety of diameters so that they can be implanted surgically.
Moreover, grafts of this type have high tensile strength in both the axial (or longitudinal) and radial direction so that the prostheses are very safe and do not dilate over time.
Grafts made of two layers of ePTFE or other plastic materials are well known in the prior art, illustrated by U.S. Pat. No. 5,800,512, PCT WO98/31305 and other references.
Generally, tubes for prior art ePTFE prostheses have been made using the following steps:
a. A PTFE resin is compounded with a lubricant (preferably a petroleum distillate, such as naphtha);
b. The compound is compacted under pressure;
c. The compacted mass is extruded into a tube using a standard ram extrusion process to its predetermined working diameter;
d. The tube is dried to remove the lubricant;
e. The dried tube is stretched longitudinally by up to 1000%;
f. The longitudinally stretched tube is sintered or cured at high temperature while its ends are fixed to insure that the tube does not shrink to its original length.
A problem with the process for manufacturing grafts in this manner is that there is a narrow range of reduction ratio that produce acceptable results. The reduction ratio is the radio of the cross-sectional area of the compacted mass to the cross-sectional area of the extruded material. If the reduction ratio is too low, the product will not have adequate strength for use as an implant. If the reduction ratio is too high, the pressure in the extruder will exceed safe manufacturing limits.
Large diameter prostheses with a wall thickness similar to that of natural vessel can be produced according to the prior art, but the resulting product is very weak because of the low reduction ratio. However, there is a need for strong, large diameter materials for surgical repair of larger vessels, such as the aorta. Furthermore, large diameter prostheses with thinner walls, which have more acceptable reduction ratios, are very difficult to produce according to the prior art because the extruded material is too fragile to be handled during the drying, expansion and sintering stages. However, there is a need for such large diameter, thin walled material for use in creating stent grafts for endovascular repair of large diameter vessels. Moreover, small diameter, thin walled material cannot be produced by the prior art because of high reduction ratio of this material. This material is needed for creating stent grafts for endovascular repair of smaller vessels, including the carotid, femoral and renal arteries.
Accordingly, it is an objective of the present invention to provide a method of making ePTFE prostheses with relatively large diameters and/or thin walls as compared to prior art prostheses.
A further objective is to provide composite prostheses made of at least two layers of a plastic material, such as ePTFE, using the inventive method.
Yet a further objective is to provide a high strength graft suitable for a stent liner.
Yet a further objective is to provide a composite device combining the high strength material with a stent.
Other objectives and advantages of the invention shall become apparent from the following description.
Briefly, a prosthesis particularly suitable for vascular reconstruction and repair, is made by first extruding an initial PTFE tube by using a ram extruder and expanding and sintering it according to the prior art. The resulting initial tube having a diameter of less than about 8 mm. is then dilated radially. The process of dilation involves expanding the tube radially by small, incremental amounts under controlled conditions until a preselected diameter is obtained. Each radial expansion is followed by calendering the tube. This process of progressive radial dilation and calendering results in a thin walled tube with exceptional strength. When the tube is heated to about 200xc2x0 C., it contracts slightly to a smaller diameter.
Using this process, two or more tubes made in the manner described may be superimposed and sintered together. First, the inner tube is placed over an appropriately sized mandrel. A second, slightly oversized tube is then placed over the first. The two tubes are than heated at approximately 200xc2x0 C., causing the inner tube to conform tightly to the mandrel, and the outer tube to contract tightly about the inner tube. The tubes, still on the mandrel, can then be sintered at high temperature to cause them to adhere to each other. In one embodiment, the laminate thus formed is used as a stent liner. In another embodiment, a stent graft is formed by inserting one or more stents between the tubes prior to sintering.
In this application, the preferred stents are made of a material exhibiting martensitic charateristics, such as Nitinol (a nickel-titanium alloy). For example, a nitinol wire can be formed into open or closed circumferential loops defining a cylindrical shape. The cylindrical shape has a stent diameter which is normally larger than the diameter of the graft when the stent is free standing. When the stent is encapsulated in the graft, it is stressed radially inward. As a result, the stent graft is biased toward a cylindrical configuration having a diameter defined by the diameter of the tubes.
This type of stent graft is first introduced into a narrow sheath having a size between 6 and 20 French. The sheath is then inserted into the desired blood vessel, the device is ejected into the vessel, and because of the biasing imparted to it by the stent, the graft opens toward its original shape, thereby engaging the sidewalls of the respective blood vessel. Stent grafts of this type are particularly useful in vessels subject to external compression, such as the femoral or carotid artery, since the device will return to its original shape after the compressive force is removed.
In addition, a single tube produced according to this method can be used to produce a stent graft. First, a stent is placed over an appropriately sized mandrel. A slightly oversized tube is then placed over the stent. The stent and tube are then heated at approximately 200xc2x0 C., causing the tube to contract tightly about the stent.
In this application, the preferred stents are made of a plastically deformable material such as stainless steel. For example, a stainless steel wire can be formed into open or closed circumferential loops defining a cylindrical shape. The cylindrical shape has a small compressed diameter but a larger expanded diameter.
This type of stent graft is positioned on a balloon catheter prior to insertion in the patient. The balloon catheter, or other expansion means, with the superimposed stent graft, is introduced into the patient by means of a narrow sheath having a size of 6 or 7 French. The balloon catheter is advanced to the delivery site, and the balloon is inflated. The stent graft opens to its expanded shape and is plastically deformed, thereby engaging the sidewalls of the respective blood vessel. Stent grafts of this type are particularly useful in smaller vessels, such as coronary arteries, since the smaller diameter of the device at introduction is critical for its clinical usefulness. Such stent grafts are also useful when the final expanded diameter of the stent graft is not precisely known prior to implantation.